[OpenSSL] Enabling Arm64 assembly on Windows by using clang

[eukarpov]

Improving OpenSSL performance on Windows by using Arm64 assembly with clang-cl

Extra Arm64 assembly will be enabled in the next stable OpenSSL build

• Enable extra Arm64 optimization on Windows for GHASH, RAND and AES
  openssl/openssl@636ee1d

[dg0yt]

AFAICT the vcpkg owners do not want to use clang as a C compiler in triplet which are meant to use MSVC as a C compiler.
(However, using clang as an assembler is accepted.)

[Neumann-A]

You also need to check that the toolchain has not a valid ASM_COMPILER being set.

[eukarpov]

OpenSSL provides two options to build Arm64 assembly with clang-cl, VC-WIN64-CLANGASM-ARM and VC-CLANG-WIN64-CLANGASM-ARM.
https://github.com/openssl/openssl/blob/master/NOTES-WINDOWS.md

In this PR, VC-CLANG-WIN64-CLANGASM-ARM is used. It provides better performance and uses clang-cl as both compiler and assembler.

@Neumann-A could you please clarify how valid ASM_COMPILER should be used if we need clang-cl for AS in any case?

[dg0yt]

In this PR, VC-CLANG-WIN64-CLANGASM-ARM is used. It provides better performance and uses clang-cl as both compiler and assembler.

The compiler must follow the toolchain. The port must not make own decisions, no matter if crypto or video.

[Neumann-A]

could you please clarify how valid ASM_COMPILER should be used if we need clang-cl for AS in any case?

If my toolchain specifies a valid CMAKE_ASM_COMPILER (which is not cl) i expect it to be used together with CMAKE_ASM_FLAGS(_<CONFIG>). The toolchain is the king deciding what to use.

[eukarpov]

It looks like NASM was used earlier without CMAKE_ASM_COMPILER check, should it be also improved?

[dg0yt]

OpenSSL uses a proprietary configuration/build system. Feeding in our precise toolchain settings might be a different task.

But selecting the best-matching configuration identifier is a fair request. VC-CLANG-WIN64-CLANGASM-ARM is for clang triplets. VC-WIN64-CLANGASM-ARM seems to be an acceptable option for the standard windows triplets.

This means you need to detect the C compiler, and you need to find/acquire the suitable clang as an assembler.

[eukarpov]

It has been changed as it was suggested

[Cheney-W]

Don't forget modify the vcpkg/ports/openssl/vcpkg.json file:

{
  "name": "openssl",
  "version": "3.1.3",
  "port-version": 1,

[data-queue]

Does this mean if the C compiler does not match clang we are still assuming Clang is the compiler? Since Clang is in both variable names

[eukarpov]

clang-cl is used as the assembler in both cases, with clang as the C compiler or with some other C compiler.